The invention relates to a heatable roll for a paper machine, paper finishing machine, or equivalent. The roll is heated by a heating medium which is introduced into the roll interior through at least one of the ends of the roll. The heating medium acts upon the material of the roll mantle, or the material of the roll, and is arranged to flow across the axial length of the roll. Thereafter, the heating medium is arranged to flow out of the roll through either one of the ends of the roll, i.e. the same end through which the heating medium entered into the roll or an opposite end.
The invention also relates to a method for heating a roll for use in paper machines, paper finishing machines or other paper machines. A heat transfer medium is introduced into a roll, circulated through the roll and removed from the roll. In this manner, the material of the roll mantle or the material of the roll is heated.
Further, the invention also relates to a method for maintaining a substantially constant temperature on an outer surface of the roll over which a paper web or board will pass.
In paper machines and paper finishing machines, in particular in calenders and super-calenders, heatable rolls are commonly used. The rolls are heated by means of a heat-transfer medium, such as hot water or oil.
There are mainly two different types of heatable rolls in the prior art. The first type of heatable rolls have a roll mantle, or are massive rolls, wherein substantially axial bores are formed in proximity to the outer face of the roll. The heating medium is made to flow through the bores from one end of the roll to an opposite end of the roll. Generally, a number of such bores are provided in the roll and are uniformly spaced in the direction of the circumference of the roll. The heating medium may be arranged to circulate in the bores either once in a direction from one end of the roll to the other, or twice, or even several times, so that in adjacent bores the heating medium flows in opposite directions. One such so-called "drilled roll" has been described earlier, e.g., in published European Patent Application No. EP-0 158 220.
On the other hand, a second type of heatable roll is a so-called double-mantle roll or rolls provided with an interior piece. This type of heatable roll is commonly used in paper machines. In this type of roll, an interior piece is fitted inside the roll mantle so that an annular intermediate space remains between the interior piece and an inner face of the roll mantle. The heating medium circulates in the annular space from one end of the roll to the other end of the roll. One such roll provided with an interior piece is described, e.g., in Finnish Patent No. 74,069.
A problem in prior art heatable rolls is that owing to the construction of the rolls, the profiles of the surface temperature in the rolls are almost always uneven. The rising differences in temperature in the axial direction of the roll are influenced by the construction and size of the roll. In rolls provided with interior pieces, typical differences in the surface temperature, on the surface over which the web runs, in the axial direction of the roll are in the range about 3.degree. C. to about 6.degree. C. On the other hand, in drilled rolls, a typical reduction of the surface temperature between the ends of the bores in the roll is in the range of about 3.degree. C. while the maximum difference in temperature in the axial direction of the roll is in the range of about 9.degree. C. and the difference in temperature in a cross-sectional plane of the roll is in the range of about 6.degree. C.
The temperature differences in both types of prior art rolls produce dangerous and very detrimental thermal strains in the roll. Deformations which can be noticed in the smoothness of the paper, and which deteriorate the runnability of the machine, are also caused by such temperature differences. Therefore, a commonly imposed requirement on the variations in temperature in the working face, i.e. the outer face, of a roll is in the range of about .+-.1.5.degree. C. Thus, in prior art rolls, it is a significant drawback that the rolls have not been able to conform with this requirement.
Reference is also made to U.S. Pat. No. 4,658,486 (Schonemann) which describes a heatable calendar roll having axial passages formed in the roll mantle for circulating a heating medium. However, it is a significant drawback that the roll described in this reference does not provide a substantially uniform temperature along the axial length of the roll mantle. This is because there are no means provided to increase the coefficient of heat transfer in the roll material in the flow direction of the heating medium.